Coherent beam combining (CBC) fiber laser amplifier system

ABSTRACT

A coherently beam combining (CBC) fiber laser amplifier system including beam shaper array assembly and a beam source that provides a plurality of beams having a low fill factor profile. The assembly includes an input beam shaper array having a plurality of input cells positioned adjacent to each other that are shaped to cause the beam to expand as it propagates away from the input array to be converted from the low fill factor profile to a high fill factor profile and cause the profile to taper to a lower value at a perimeter of each input array cell. The assembly further includes an output beam shaper array having a plurality of output cells positioned adjacent to each other that are shaped to cause the beam to stop expanding so that the output array provides a plurality of adjacent beams with minimal overlap and a minimal gap between the beams.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 16/592,233, titled, Coherently Combined Fiber Laser Amplifier SystemIncluding Monolithic Phased Array With Compact Tiles, filed Oct. 3,2019.

BACKGROUND Field

This disclosure relates generally to a coherently combined fiber laseramplifier system and, more particularly, to a coherently combined fiberlaser amplifier system including a beam shaper array assembly havingspaced apart beam shaper arrays, where one of the beam shaper arraysincludes close-packed tiled beam shaper cells that each convert a lowfill factor beam to a high fill factor beam and the other beam shaperarray also includes close-packed tiled beam shaper cells that each stopthe expansion of one of the high fill factor beams.

Discussion

High power laser amplifiers have many applications, includingindustrial, commercial, military, etc. Designers of laser amplifiers arecontinuously investigating ways to increase the power of the laseramplifier for these and other applications. One known type of laseramplifier is a fiber laser amplifier that employs a doped fiber thatreceives a seed beam and a pump beam that amplifies the seed beam andgenerates the high power laser beam, where the fiber has an active corediameter of about 10-20 μm or larger. Fiber laser amplifiers are usefulas energy sources for directed energy weapons because of their highefficiency, high power scalability and excellent beam quality.

Improvements in fiber laser amplifier designs have increased the outputpower of the fiber amplifier to approach its practical power and beamquality limit. To further increase the output power some fiber lasersystems employ multiple fiber laser amplifiers that combine theamplified beams in some fashion to generate higher powers. A designchallenge for fiber laser amplifier systems of this type, especiallythose employed in directed energy weapons that direct a high energy beamon a target, is to combine the beams from a plurality of fiberamplifiers in a manner so that the beams provide a single beam outputhaving a uniform phase over the beam diameter such that the beam can befocused to a small focal spot. Focusing the combined beam to a smallspot at a long distance (far-field) defines the quality of the beam.

There are two approaches to scaling beam combiner laser weapons systemsto higher powers. One approach is known as spectral beam combining(SBC), where multiple lasers of different wavelengths are combined on adiffraction grating or other dispersive optic into a single beam. Theother approach is known as coherent beam combining (CBC), where multiplemutually coherent lasers are locked in phase with one another andcombined into a single beam either by overlapping in the near fieldusing a beam splitter, or by tiling side by side to form a compositebeam, a configuration that is colloquially referred to as a “phasedarray”.

Of the different beam combining approaches, the phased array approach isunique in that it provides added utility beyond simply higher power withgood beam quality. By changing the relative phases (“piston”) betweenthe side-by-side laser tiles, a composite wavefront across the tiledbeam can be synthesized. This synthesized wavefront can provide eitherhigh speed beam steering by applying a linear phase ramp across thearray elements, or more generally can compensate any downstreamwavefront aberration by imposing the conjugate wavefront on thecomposite tiled beam. This can enable aim-point maintenance on a highspeed moving target despite high platform jitter, and can also enablethe formation of a near diffraction limited spot on a distant targetdespite large intervening atmospheric turbulence and aero-opticaberrations. In this regard, the capability is essentially similar tothat of a beam control system using a traditional deformable mirror(DM), but without the need for DM hardware and with orders of magnitudefaster actuation capability. DMs are typically limited in theiractuation speed to acoustic-class (multi-kHz) speeds due to the need tophysically deform a mirror surface, while phased array pistons can beactuated at GHz-class speeds using commercially fiber-coupled waveguideelectro-optic modulators. These high speeds can be desirable for activecompensation of rapidly varying wavefront disturbances, particularlythose arising from aero-optic effects on moving air platforms.

It is desirable in these types of fiber array amplifiers to provide anarray of fiber beams having a high spatial fill factor, which forflat-top beams is defined as the fraction of the combined beam areaoccupied by the high power beams. For the case of beams withnon-flat-top profiles, the fill factor can be more generally defined as1−σp²/(4P²), where P is the average laser intensity and σp² is thevariance of the laser intensity across the combined beam area. The fiberbeam emitters in these systems emit a beam typically having a roundnear-Gaussian profile, although other non-Gaussian profiles arepossible, and the beams are arranged in an array next to each other. Thefiber beams are then collimated by collimating optics, where spacesbetween the beams cause optical power to be generated in spatialsidelobes that are likely to be off-target in the far-field. Therefore,it is desirable to fill the entire aperture of the telescope thatprojects the combined beam on the target by increasing the fill factorto obtain the smallest possible spot in the far-field. The known fiberamplifier systems typically employ lenses that collimate the roundGaussian beams to increase the fill factor. However, high fill factorbeam arrays whose elements have near-Gaussian profiles exhibit highclipping losses since the wings of the beams are blocked by neighboringelements. Thus, there is a need for beam shaper arrays with higher fillfactors and lower clipping losses than is possible using simple lensesand near-Gaussian beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a CBC fiber laser amplifiersystem that includes a beam shaper array assembly;

FIG. 2 is an isometric type view of the beam shaper array assemblyseparated from the fiber laser amplifier system shown in FIG. 1 ;

FIG. 3 is a front view of a beam shaper array in the beam shaper arrayassembly shown in FIG. 2 ;

FIGS. 4A-4F are beam profiles for one of the fiber beams propagatingthrough the beam shaper array assembly shown in FIG. 2 showing how thebeam profile evolves and expands as it propagates from one beam shaperarray to the other beam shaper array to go from a round Gaussian profileto a square flat-top profile;

FIG. 5 is an illustration of one of the beam shapers in the beam shaperarray shown in FIG. 3 that is shaped in both the X and Y directions;

FIG. 6 is an illustration of one of the beam shapers in the beam shaperarray shown in FIG. 3 shaped in the X direction;

FIG. 7 is an illustration of one of the beam shapers in the beam shaperarray shown in FIG. 3 shaped in the Y direction;

FIG. 8 is an isometric type view of the beam shaper array assembly shownin FIG. 2 and including trim beam shaper arrays;

FIG. 9 is an isometric type view of a beam shaper array assembly of thetype shown in FIG. 2 and including beam shaper arrays on opposing sidesof a block;

FIG. 10 is a front view of a beam shaper array having a number ofseparate square beam cells where the cells have been tiled so that anouter edge of the beam shaper array more closely matches a roundaperture;

FIG. 11 is a front view of a beam shaper array having beam cells ofdifferent shapes;

FIG. 12 is a front view of a beam shaper array having beam cells ofdifferent sizes;

FIG. 13 is a front view of a beam shaper array having beam cells ofdifferent sizes and shapes for a round aperture;

FIG. 14 is an isometric type view of a beam shaper array assembly of thetype shown in FIG. 2 and including a diode laser bar beam source;

FIG. 15 is a schematic block diagram of an SBC fiber laser amplifiersystem that includes a one-dimensional beam shaper array assemblysimilar to the beam shaper array assembly shown in FIG. 2 ;

FIG. 16 is an isometric view of the one-dimensional beam shaper arrayassembly separated from the amplifier system shown in FIG. 15 ;

FIG. 17 is a schematic block diagram of a hybrid CBC and SBC fiber laseramplifier system that includes a beam shaper array assembly of the typeshown in FIG. 2 and includes multi-dithered sample beam phase-locking;

FIG. 18 is an isometric view of an endcap for the beam shaper arrayassembly shown in FIG. 17 ;

FIG. 19 is a schematic block diagram of another hybrid CBC and SBC fiberlaser amplifier system that includes a beam shaper array assembly of thetype shown in FIG. 2 and includes optical heterodyne detectionphase-locking;

FIG. 20 is a flow chart diagram showing a process for process forfabricating a beam shaper array assembly;

FIG. 21 is an illustration of a 1D optical assembly showing a processfor welding fibers to an endcap and measuring beam angle and positionerrors using a telescope and microscope;

FIG. 22 is a front view of a near-field image showing beam positionerrors;

FIG. 23 is a front view of a far-field image showing beam angle errors;

FIG. 24 is an illustration of a 1D optical assembly showing a processfor measuring beam angle and position errors for a beam shaper arrayassembly using optical elements to image beams in the far-field and thenear-field;

FIG. 25 is an illustration of a 1D optical assembly showing a processfor measuring beam angle and position errors for a beam shaper arrayassembly using an interferometer;

FIG. 26 is a front view of an image at the interferometer in FIG. 25 ;

FIGS. 27A-27D are side views of an emitter array showing a process forcorrecting an angle of a fiber in the array;

FIGS. 28A-28C are side views of an emitter array showing a process forcorrecting a position of a fiber in the array;

FIG. 29 is an illustration of the optical assembly showing an endcapbeing coupled to the optical element by opposing side bridges;

FIG. 30 is an illustration of the optical assembly showing an endcapbeing coupled to the optical element by a top bridge;

FIG. 31 is an illustration of a 2D optical assembly including aplurality of 1D optical assemblies and optical element; and

FIG. 32 is an illustration of a 2D optical assembly including aplurality of 1D optical assemblies and a single optical element.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the disclosure directedto a coherently combined fiber laser amplifier system including a beamshaper array assembly including spaced apart tiled beam shaper arrays ismerely exemplary in nature, and is in no way intended to limit thedisclosure or its applications or uses.

FIG. 1 is a schematic block diagram of a CBC fiber laser amplifiersystem 10 that includes a seed beam source 12 that generates acontinuous wave frequency-modulated seed beam having a center wavelength2 on a fiber 14. The source 12 may include a master oscillator (MO),such as a single-longitudinal mode distributed feedback (DFB) diodelaser oscillator, and a frequency modulator, such as an electro-opticalmodulator (EOM). The EOM may receive an applied voltage provided by anamplified radio frequency (RF) electrical drive signal from an RF source(not shown) that provides frequency modulation broadening, such as whitenoise or pseudo-random bit sequence (PRBS), so that the modulated seedbeam has a linewidth that is substantially broadened, which suppressesstimulated Brillouin scattering in a downstream high power fiberamplifier. The modulated seed beam on the fiber 14 is split by anoptical splitter 16 to produce a plurality of split seed beams on fibers18 having the same wavelength λ, where each split seed beam is sent to aseparate EOM 20 that provides servo-phase control of the seed beams forphase-locking purposes.

Each of the modulated seed beams on the fibers 22 is sent to a fiberamplifier 24, such as a Yb-doped fiber amplifier, where the amplifier 24will typically be a doped amplifying portion of the fiber 22 thatreceives an optical pump beam (not shown). All of the amplified beamsare directed onto fibers 26 and sent to a beam shaper array assembly 40,discussed in detail below, that operates as a system emitter. Theemitted amplified beams from the beam shaper array assembly 40 aredirected as a combined amplified beam through a beam splitter 28 in aphase sensing assembly 38 that samples off a sample portion of each ofthe separate beams in the combined beam. The optical splitter 16 isconfigured to also generate a reference beam 58 that is sent to areference beam modulator 178 that modulates the reference beam 58. Thereference beam modulator 178 could include, for example, anacousto-optic modulator that shifts the center frequency of thereference beam 58, or an EOM that imparts a digital phase shift to thereference beam 58. The modulated reference beam 58 is expanded by a beamexpander 78 to provide a flat wavefront and to overlap with the combinedamplified beam on the beam splitter 28. The intensities of theoverlapped reference and sample beams are detected by an array 30 ofphotodetectors 32. The main part of the combined amplified beam is sentto a beam director telescope 34 that directs an output beam 54 to atarget (not shown).

The electrical signals from the photodetectors 32 are used by a phaselocking controller 36 to control the EOMs 20 to correct the phase of theseed beams, using, for example, a phase-locking technique, such asoptical heterodyne detection (OHD), well known to those skilled in theart. However, other phase-locking techniques (not shown) can be employedthat may not require a frequency shifted reference beam, or insteadusing a far-field generating lens that focuses the entire beam arrayonto a single detector, where error signals for each channel areextracted electrically using a variety of multi-dither approaches, suchas, for example, a stochastic parallel gradient decent (SPGD) algorithm,well known to those skilled in the art. The controller 36 may receiveother data and information provided by box 58 to determine the phaseset-points, such as wavefront aberration data or beam steeringset-points. More particularly, the controller 36 provides error signalsof the phase difference between the sampled beams, and provides thoseerror signals to the EOMs 20 to control the phases of the individualseed beams so that all of the seed beams are locked in phase. In otherwords, the EOMs 20 provide seed beam phase control so that a “piston”phase of the combined amplified beam is spatially uniform across thebeam wavefront. This also allows the phases of the seed beams to bealtered relative to each other for electronic beam steering purposes.The controller 36 can also impart phase control of the beams to correctfor measured atmospheric anomalies where the output beam 54 may havewavefront aberrations that are corrected as a result of propagatingthrough the atmospheric aberrations so that the beam 54 is of thedesired quality when it impinges the target.

FIG. 2 is an isometric type view of the beam shaper array assembly 40separated from the fiber laser amplifier system 10. As will bediscussed, the array assembly 40 provides nearly a 100% fill factor withuniform or tailored beam intensity across the emitting aperture of thetelescope 34 with a uniform or tailored phase or phase front. The phasesmay be tailored for beam steering or compensation of aberrations asdescribed above. The array assembly 40 includes an optical endcap 42that is a monolithic block of optical glass, or a group of monolithicblocks, where ends of the fibers 26 are optically welded to an inputsurface 46 of the endcap 42 by suitable stems 44 that are machined intothe surface 46 so that the amplified beams are emitted into the endcap42 with minimal reflections and are precisely spaced apart withmicrometer-class precision. The array assembly 40 also includes a pairof spaced apart tiled beam shaper arrays 48 and 50, where each of thebeam shaper arrays 48 and 50 includes an array of close-packed tiledcells having beam shapers, one for each beam, that changes the spatialphase distribution, or wavefront, of the beam so that it is shaped in adesired manner.

FIG. 3 is a front view of the beam shaper array 48 showing an array ofsquare tiled sub-aperture beam shaper cells 56 each having a beam shaper52 with a surface that is shaped so that the beam is focused in themanner discussed herein, where after propagation from the beam shaperarray 48 to the beam shaper array 50 the beams are altered from a roundGaussian profile to a square flat-top profile having the size of thecell 56. In other words, the beam shaper array 48 changes the phase ofthe beams so that as they propagate the beams flatten out and fill inspaces between the beams with light. The beam shaper array 50 includesthe same configuration of cells and beam shapers as the beam shaperarray 48, but the beam shapers are shaped differently so that instead ofexpanding the beams they change the phase front of the beams to maintainthe beams in the flat-top profile as they further propagate. In thisnon-limiting embodiment, the sub-aperture size provided by one of thecells 56 is 2×2 mm².

As mentioned, the beam shaper array 48 operates to change the shape ofthe round Gaussian-shaped amplified beams into square flat-top beams,where the square flat-top beams are positioned very close together,i.e., tiled and closely-packed, over a certain predetermined propagationdistance so that there are minimal gaps between the beams to provide thedesired near-100% fill factor. In other words, the Gaussian-shaped beamsfrom the fibers 26 are positioned next to each other in atwo-dimensional array by the endcap 42, and then the low intensity areasbetween the Gaussian peaks are filled in by the beam shaper array 48upon propagation from the beam shaper array 48 to the beam shaper array50 so that the beam intensity across the array of beams is constant witha minimal overlap and a minimal gap between the beams. The beam shaperarray 50 is positioned at the predetermined distance from the beamshaper array 48 and operates to stop the shape changing of the beams sothat the output beams from the beam shaper array 50 have a flat phasefront.

FIGS. 4A-4F are beam profiles for one of the amplified beams showing howthe beam profile evolves and expands as it propagates from the beamshaper array 48 to the beam shaper array 50 to go from the roundGaussian profile to a square flat-top profile. Particularly, FIG. 4Ashows the profile of the beam at the output of the beam shaper array 48as having a Gaussian profile, FIG. 4B shows the profile of the beam at10 mm from the array 48, FIG. 4C shows the profile of the beam at 20 mmfrom the array 48, FIG. 4D shows the profile of the beam at 30 mm fromthe array 48, FIG. 4E shows the profile of the beam at 40 mm from thearray 48, and FIG. 4F shows the profile of the beam at 50 mm from thearray 48 at the location of the beam shaper array 50. The beam shaperarray 50 then conjugates or collimates the spread out beam to remove theresidual phase and divergence of the beam to provide the square flatwavefront beam. Because there is no or minimal overlap of the beams incontiguous sub-apertures, electronic beam steering is available bychanging the phase of the various beams. In other words, separate beamfootprints preserve the full adaptive optics capability of the fiberlaser amplifier system 10.

The beam shapers 52 are shaped in both the X and Y directions relativeto the Z propagation direction of the beams to obtain the necessaryshape to flatten and make square the beam profile upon propagation fromthe array 48 to the array 50. One suitable shape of the beam shapers 52is shown in FIG. 5 , where the shape is formed on one surface. In thisembodiment, the substrate material of the beam shaper 52 is fusedsilica, the X and Y axes each span 2 mm, and the Z axis shows thesurface relief in micrometers. Alternately, separate surfaces can beshaped for the X and Y directions of the shape of the beam shaper. Forexample, FIG. 6 shows a suitable shape for the X direction and FIG. 7shows a suitable shape for the Y direction. The separate X and Ydirection shapes can be formed on opposite sides of the same beam shaperarray or can be on one side of two adjacent beam shaper arrays. Theseembodiments with separated X and Y direction shapes are advantageous incomparison to combined X and Y direction shapes because they reduce themaximum surface relief of the beam shaper arrays 48 and 50, thus makingthem easier to fabricate with high accuracy.

In this non-limiting embodiment, the beam shaper array 48 is shownspaced apart from an output surface of the endcap 42. However, the beamshaper array 48 could be optically attached to or shaped directly on anoutput surface of the endcap 42. It is necessary to provide someeffective free space distance between the tip of the fibers 26 welded tothe input surface 46 and the beam shaper array 48 to allow for some beamexpansion. As shown in FIG. 1 , this effective free space could becomprised partially or entirely of propagation within a glass mediumwith appropriate consideration of the medium's index of refraction. Theamount of effective free space between the fiber tips and the beamshaper array 48 depends on the sub-aperture pitch (spacing) between thefibers 26, the numerical aperture, i.e., divergence, of the Gaussianbeam emitted from each fiber 26, and the aperture of the telescope 34,with the goal being to obtain the beam footprint size at the beam shaperarray 48 to be an appreciable fraction of the pitch, but be fullycontained within the sub-aperture without clipping losses. Thus, for atypical large mode area fiber having a numerical aperture, i.e.,half-angle divergence, of 0.03 radians and for a pitch of 2 mm, theeffective free space distance between the fiber tips and the beam shaperarray 48 would be about 10 mm.

Although the pair of beam shaper arrays 48 and 50 should in theory beable to provide the desired beam shaping as described, in practice thereis likely to be various wavefront aberrations and errors in the beamintensity profile at the output of the beam shaper array 50 caused bymanufacturing tolerances of the assembly 40, which may in turn be drivenin part by manufacturing tolerances of the arrays 48 and 50 and theendcap 42, and their relative alignments, which may be able to bemeasured for a specific assembled set of components.

FIG. 8 is an isometric type view of the beam shaper array assembly 40separated from the fiber laser amplifier system 10, where a trim beamshaper array 62 is provided adjacent to the beam shaper array 50 thatincludes an array of trim beam shaper elements 64 that are shaped toprovide wavefront corrections based on the measurements to provide thereverse of the aberrations. Each of the trim beam shaper array elements64 within the array 62 may be different from the others based onmeasured errors. The assembly 40 also includes a trim beam shaper array66 that is provided adjacent to the beam shaper array 48 that includesan array of trim beam shaper elements 68 that are shaped so that afterpropagation of the beams from the beam shaper array 48 to the beamshaper array 50 any non-uniformity in the intensity distributionmeasured at the output of the beam shaper array 50 will be corrected.Each of the elements 68 within the array 66 may be different from theothers based on measured errors. Alternately, or in addition, thewavefront aberrations of individual components, such as, for example,the arrays 48 and 50, or an assembled fiber endcap array comprising thefibers 26, the connectors 44 and the endcap 42 can be measuredseparately and the trim beam shaper arrays 62 and 66 can also beprovided adjacent to the arrays 48 and 50, respectively.

In the embodiment discussed above, the beam shaper arrays 48 and 50 arediscrete elements. In an alternate embodiment, the two beam shaperarrays can be provided on opposing faces of a single optical block thathas the necessary thickness so that the low fill factor beams areconverted to the flat-top high fill factor beams as the beams propagatethrough the block. This embodiment is illustrated in FIG. 9 showing aside view of a beam shaper array assembly 70 including an optical block72, an input beam shaper array 74 optically adhered to or shapeddirectly on an input surface of the block 72 and an output beam shaperarray 76 optically adhered to or shaped directly on an output surface ofthe block 72, where the beam shaper array 74 would be spaced from orattached to the endcap 42 to provide an effective free space distancebetween the tips of the fibers 26 welded to the input surface 46 and thebeam shaper array 74. Similarly, the beam shaper array 48 could beshaped directly on the surface of the endcap 42. These embodiments areadvantageous because they reduce the number of surfaces the high powerbeams pass through, thus reducing optical power losses from unwantedscattering or back reflections.

In the above embodiment, the beam shaper arrays 48 and 50 are square. Inother embodiments, it may be desirable to maintain the square tiling ofthe individual beam shapers and beams, but make the outer shape of thecombined beam to more closely match the shape of the aperture of thetelescope 34. FIG. 10 is a front view of a beam shaper array 80 that canbe used as the arrays 48 and 50 and having a number of square beamshaper cells 82 including beam shapers, where the cells 82 are tiled sothat an outer edge 86 of the beam shaper array 80 more closely matches around aperture. Further, in certain fiber amplifier systems of the typedescribed herein, the main aperture of the telescope 34 may have acenter obscuration, such as for accommodating a secondary mirror. Inorder to address this issue, the beam shaper array 80 can have a centeropening 88 where no beams would propagate so as to not waste beam powerthat would otherwise impinge the obscuration.

Although it has been suggested above that all of the cells 56 have thesame square shape, in other designs it may be desirable to control thepower or intensity of the combined beam in different areas, such as atthe center, where the outer cells may have a different shape that theinner cells, such as some cells being square and other cells beingrectangular, to adjust the beam power accordingly. This embodiment isillustrated in FIG. 11 showing a front view of a beam shaper array 90that can be used as the arrays 48 and 50 and having a number of squareinner beam shaper cells 92 having beam shapers 94 and a number ofrectangular outer beam shaper cells 96 having beam shapers 98.

Also, the cells 56 do not all need to be the same size, where differentsized cells may better accommodate an outer or inner circumferenceshape, such as shown in FIG. 10 . This embodiment is illustrated in FIG.12 showing a front view of a beam shaper array 100 that can be used asthe arrays 48 and 50 and having a number of square beam shaper cells 102with beam shapers 104 of one size and a number of square beam shapercells 106 with beam shapers 108 of another size.

Although it has also been suggested above that all of the cells 56 arearranged in a rectilinear array, in other designs it may be desirable toarrange the cells in an array with non-rectilinear shapes. For example,a close-packed hexagonal array of hexagonally shaped cells may beadvantageous to better fill a circular beam director aperture than arectilinearly shaped cell. Furthermore, a more general application ofvaried cell shapes may be desirable to better accommodate the apertureboundaries.

FIG. 13 is a front view of a beam shaper array 116 that can also be usedas the arrays 48 and 50 having an outer substantially circularcircumference to conform to a circular aperture 118 that illustratesthese features. The array 116 includes a variety of specially shapedbeam shaper cells including hexagonal cells 146, rectangular cells 148,trapezoidal cells 190, triangular cells 192 and diamond-shaped cells 194to provide the circular shape.

It is noted that although the above embodiments use an array of fiberemitters, alternate embodiments may use other types of laser emitters,such as diode laser emitters, where the number of emitters could besignificantly increased. More specifically diode lasers are oftenfabricated lithographically in semiconductors where the lasers can beclosely arranged. FIG. 14 is an illustration of a laser system 110 thatgeneralizes the type of lasers that can be employed, where an emitterarray 112 is provided at the input side of the beam shaper array 48 andincludes diode lasers 114.

Although it has also been suggested above that the desired outputintensity distribution after the beam shaper array 50 has a flat top foreach of the array cells, for some applications this may not bedesirable. For example, if it is desired to synthesize a tiled arraywith an intensity distribution that tapers to a low value near itsperimeter so as to minimize far-field sidelobes with the central cellsof the tiled arrays exhibiting higher intensity than the perimetercells, as would be the case, for example, using the configurations ofthe beam shaper arrays 90 and 100, then it may also be advantageous totaper the intensity distribution internal to some or all of the cellswithin the array so that their intensity is larger near the edge of thecell closest to the center of the array, and smaller near the outer edgeof the cell at the outer boundary of the array, such as the array 90, ornear the inner edge of an annular ring, such as the center opening 88.This can be accomplished by tailoring the design of the beam shapers inthe two beam shaper arrays to yield the desired tapered intensitydistribution, which can be unique for each element of the array.

A beam shaper array assembly similar to the beam shaper array assembly40, but that does not employ coherent combining by phase locking canalso be employed in SBC fiber laser amplifier architectures. FIG. 15 isa schematic block diagram of an SBC fiber laser amplifier system 120illustrating such an architecture that includes N number of wavelengthchannels 122 each having a seed beam source 124 that generates acontinuous wave frequency-modulated seed beam having a center wavelengthon a fiber 128 for the particular channel 122, where each seed beamsource 124 generates a different beam wavelength λ₁−λ_(N). Each of theseed beams on the fibers 128 is sent to a fiber amplifier 130, such as aYb-doped fiber amplifier, where the amplifier 130 will typically be adoped amplifying portion of the fiber 128 that receives an optical pumpbeam (not shown). All of the amplified beams are directed to aone-dimensional beam shaper array assembly 132 that is similar to thebeam shaper array assembly 40, but does not provide phase locking asdiscussed above because all of the beams have different wavelengths.

FIG. 16 is an isometric view of the one-dimensional beam shaper arrayassembly 132 separated from the amplifier system 120. The beam shaperarray assembly 132 is a one-dimensional array having one row of fibersalong a combining axis that provides beam shaping that may differbetween the combining dispersive axis and the orthogonal non-combining,non-dispersive axis. Along the combining axis, the beam shaping is notnecessarily intended to provide a fully filled rectangular aperture, butcan be more of a hyper-Gaussian or similar shape with a higher fillfactor than the input beam shape, and with a smoothly tapered intensityprofile. The beam shaper array assembly 132 flattens out the intensitydistribution of the beams from each fiber 128 to increase the fillfactor. More specifically, the beam shaper array assembly 132 includesan endcap 134 to which the fibers 128 are coupled, a beam shaper array136 that includes beam shapers that spread out the beam intensity in asimilar manner as the array 48 as shown in FIGS. 4A-4F, and a beamshaper array 138 that includes beam shapers that stops the beamspreading in the same manner as the array 50 so that the beam shaperarray assembly 132 outputs a set of wavelength-diverse amplified highfill factor beams into free space, where the individual beam wavelengthsλ₁−λ_(N) are propagating from slightly different emitter positions.

The high fill factor beams are reflected off of a set of collimatingoptics 140 that collimates the diverse beams and directs them onto anSBC grating 142 so that all of the individual beams impact the grating142 and overlap on the same footprint. The grating 142 spatiallydiffracts the individual beam wavelengths λ₁−λ_(N) and directs theindividual amplified beams in the same direction as a combined outputbeam 144.

SBC beam quality is limited by angular dispersion from the diffractiongrating 142. Since the individual fiber amplifiers have a finite opticallinewidth owing to frequency modulation, power will be spread intodifferent directions following diffraction from the diffraction grating142. If the span of these different directions is significant comparedto the diffraction limited angle, then the beam quality will degrade.The degradation in beam quality can be minimized by decreasing the sizeof the beam footprint on the grating 142 along the combining dispersiveaxis, which increases the diffraction limited angle. Since the grating142 is at a Fourier plane of the array 138, this corresponds to anincrease in the individual beam sizes at the array 138. Hence, a highspatial fill factor along the combining axis of the array 138 providesimproved beam quality. With an array of Gaussian beams, the fill factorat the array 138 cannot be increased without clipping the wings of theGaussian beams leading to lost power. With an array of shaped high fillfactor beams, for example, an array of hyper-Gaussian shaped beams, thespatial fill factor of the array 138 can be increased without incurringclipping losses, thus leading to improved beam quality without loss ofpower.

For the SBC configuration of the laser system 120, the shape of thebeams output from the array 138 will optimally be identical for all ofthe beams. However, the beam shapes may be different along the combiningand the non-combining axes. Along the combining axis the beams may beshaped as discussed above to provide a higher fill factor to minimizebeam quality loss due to angular dispersion. Along the non-combiningaxis, the beams may, for example, be left unshaped to generate anear-Gaussian beam profile on the grating 142. This can be advantageousto maximize power on a far-field target whose size is between 1× and 2×diffraction-limited (DL), since it minimizes power diffracted intofar-field sidelobes at angles larger than 2× diffraction-limited.Alternatively, the beams may be shaped along the non-combining axis togenerate a high fill factor profile on the grating 142, with intensitytapering to near zero at the telescope aperture. This can beadvantageous to fully fill a beam director telescope to maximizefar-field peak intensity on a target whose size is less than 1xdiffraction-limited while minimizing clipping losses on the telescopeaperture. The dispersive impact of the grating 142 can be minimized onoutput beam quality by choosing the beam shaper arrays 136 and 138 andthe set of collimating optics 140 so that the shaped beams incident onthe grating 142 are narrow in the dispersive direction, but wider in theorthogonal non-dispersive direction. This asymmetric configurationlowers the peak irradiance on the grating 142 while also minimizing thedegradation of the combined beam quality.

The number of the fibers that are coupled to the endcap 42 is generallylimited by the mechanical limitation of the ability to coupleclosely-spaced fibers to the endcap 42. The number of beams that can becombined in both SBC and CBC architectures is limited for other reasons.In particular, the one-dimensional linear fiber array required for SBCmay be impractically large. However, the limitations of providing CBCand SBC combined beams can be increased by a hybrid fiber laseramplifier architecture that combines both CBC and SBC architectures.This can be accomplished by providing CBC combined beams in onedirection and SBC combined beams in an orthogonal direction. Thiscombined CBC and SBC architecture can then be improved with the beamfill factor provided by the beam shaper array assembly 40 or the beamshaper array assembly 132 discussed above.

Several architectures can be employed to measure the beam phases in ahybrid CBC and SBC fiber laser amplifier system. FIG. 17 is a schematicblock diagram of a hybrid CBC and SBC fiber laser amplifier system 150illustrating one such architecture, where like elements to the system 10are identified by the same reference number. The system 150 includes Nnumber of SBC channels 152, where each channel 152 includes a separategrouping of the seed beam source 12 having wavelength λ_(i), where i isin the range 1≤i≥N, the splitter 16, the EOMs 20 and the amplifiers 24as shown in the system 10. As such, the ith channel 152 includes asingle wavelength λ_(i) seed beam that is split into M multiple seedbeams that are separately amplified and of the same wavelength λ_(i),where there are N groups of M EOMs 20 and all of the channels 152together generate M×N seed beams that are amplified at each of thedifferent beam wavelengths λ₁−λ_(N). All of the M×N fibers 26 arecoupled to an endcap 154 that is part of a beam shaper array assembly156 including an input beam shaper array 158 and an output beam shaperarray 160 of the type shown in FIG. 2 that outputs M×N output beams 162.

FIG. 18 is an isometric view of the endcap 154 separated from the system150. The endcap 154 includes an optical block 164 having an input side166 and an output side 168, where a non-dispersive CBC axis identifiedby arrow 172 is defined along one side of the block 164 and isperpendicular to the plane of the paper in FIG. 17 and a dispersive SBCaxis identified by arrow 170 is defined along a perpendicular side ofthe block 164 and is into the plane of the paper in FIG. 17 . The fibers26 are optically coupled to the input side 166 of the block 164 and abeam 174 from each fiber 26 is shown at the output side 168 of the block164, where the beams 174 in each row along the CBC axis 172 have thesame wavelength λ_(i) and are provided by one of the channels 152, andthe beams 174 in each column along the SBC axis 170 comprise themultiple wavelengths λ₁− λ_(N) and are provided by different ones of thechannels 152. The beam shaper arrays 158 and 160 that receive the beamswill have cells and beam shapers configured in the manner discussedabove so that phase-locking of the beams occurs along the CBC axis 172and beam intensity flattening occurs to increase the fill factor alongthe SBC axis 170.

The M×N beams 162 from the beam shaper array 160 are collimated by acylindrical optical system 196 and directed onto a diffraction grating198 that operates in a similar manner to the diffraction grating 142.The optical system 196 has curvature along the dispersive SBC axis 170in the plane of the page. The focal length of the optical system 196 isselected to ensure that all of the N different wavelengths λ_(i) areincident at the correct angles to the grating 198 such that all of thediffracted output beams from the grating 198 are co-propagating in thesame direction with the highest precision possible. As a result, acombined output beam 176 from the grating 198 comprises M parallel beamstiled along the CBC axis 172, which is orthogonal to the page, and willhave phase control of the individual beams along one axis and spatiallydiffracted beams along a perpendicular axis.

Also reflected from the grating 198 is a weak specular 0^(th) order beam180, which is focused by a lens 182 onto a detector array 184 havingindividual detectors 186. Because the wavelength groups comprising thebeam 180 propagate at different angles in a linear array, the focusedbeams from the lens 182 for each wavelength group are separated along aline and can be directed to a linear array of the detectors 186. Each ofthe N detectors 186 receives the overlap of the M focused CBC beamscomprising each wavelength group. The intensity on each detector 186 ismaximized by a multi-dither processor 188 that provides dither signalssuperimposed with phase-locking control signals to the correspondingEOMs 20 for each wavelength group using, for example, an SPGD algorithm.

FIG. 19 is a schematic block diagram of another hybrid CBC and SBC fiberlaser amplifier system 200 that is similar to the amplifier system 150,except for phase sensing and control features, where like elements tothe system 150 are identified by the same reference number. In thisembodiment, the splitters 16 provide a reference beam 202 to be used asa reference for each wavelength group. Each reference beam 202 ismodulated by a modulator 204, and all of the modulated reference beams202 are combined by a wavelength division multiplexer (WDM) 206 onto asingle fiber 208, and then collimated by a lens 210 to form a largemulti-color planar reference beam 212. The planar reference beam 212 iscombined by sample optics 216 with a small sample of the M×N beams 162transmitted from the beam shaper array 160.

The combined reference beam 212 and the sampled M×N beams 162 arereceived by a 2D M×N detector array 218 including individual detectors220, where wavelength filters (not shown) may be employed in the array218 to eliminate noise from the reference beams 212 having wavelengthsother that the correct wavelength λ_(i) intended for a given detector220. Alternately, the detector array 218 can be AC-coupled to reject DCphotocurrent arising from the reference beams having wavelengths otherthan the correct wavelength intended for a given detector 220. Theheterodyne interference signal from each detector 220 in the array 218is transmitted to an OHD processor 222 that provides phase correctionsignals to the EOMs 20 to phase lock each group of M beams at eachwavelength λ_(i).

It is noted that although the amplifier systems 150 and 200 providespecific phase control approaches, this is non-limiting in that otherapproaches may be suitable. The essential element in the hybrid fiberlaser amplifier systems 150 and 200 is that the CBC beams need to bephase locked together with the desired phase profile similar to thesystem 10. The essential difference in the hybrid systems 150 and 200 isthat the CBC beam is only in one dimension, since the beams in the otherdimension are spectrally combined.

It was mentioned above that the beam shaper array assembly 40 may not bemanufactured with enough accuracy to prevent significant and variouswavefront aberrations and errors from occurring in the beam because itis very difficult to obtain the necessary optical alignment precisionfor the beam shaper array assembly 40 in the various embodimentsdiscussed above. For example, the known fabrication processes forwelding the fibers 26 to the stems 44 is generally not accurate enoughto completely prevent both position and angle errors, which leads tobeam deviations. Additional errors that could occur at lens fabricationand integration from mechanical tolerances also leads to launcher arraybeam deviations. Errors can generally be corrected at three differentlocations along the optical path, namely, changing the weld jointbetween the fibers 26 and the stems 44, changing the beam shaper arrayassembly 40 and providing trim plates, such as the trim beam shaperarrays 62 and 66. One way to correct these wavefront aberrations is tomeasure the aberrations and provide the trim beam shaper arrays 62 and66, as discussed above. However, other techniques can also be employedto correct the aberrations. A detailed discussion of some of thesetechniques is discussed below.

FIG. 20 is a flow chart diagram 230 outlining the general process offabricating and correcting an optical element, and in particular thebeam shaper array assembly 40. At box 232, the optical element, forexample, the assembly 40, is fabricated using an inexpensive and lowprecision fabrication process where significant manufacturing errorslikely will be present in the assembly 40. During this process step forfabricating the assembly 40, the endcap 42 with the stems 44 is providedand the fibers 26 are optically welded to the stems 44 to form anemitter array, such as a 1D emitter array. A number of the 1D emitterarrays can be assembled together to form, for example, a 2D emitterarray. The 1D or 2D emitter array is then integrated with, for example,the beam shaper arrays 48 and 50. At box 234, wavefront errors causedby, for example, the assembly 40 are measured using, for example, anoptical system that creates an image relay onto a sensor of thedistribution from the plane of the beam shaper array 48, the plane ofthe beam shaper array 50 and the far-field plane, where the sensors canbe a camera for measuring irradiance distributions or an interferometerfor measuring wavefront distributions. At box 236, calculations are madefor prescriptions and remedies that are used to correct the errors. Atbox 238, a suitable process is performed to correct the errors based onthe prescriptions, such as remanufacturing the beam shaper arrayassembly 40, providing stand-alone optical trim plates, changing thefiber emitter position and angle trimming, etc. If additionalcorrections are required, then the process returns to the box 234 toagain measure the wavefront errors. If the corrections are successful,the beam shaper array assembly 40 is accurate and operational at box240.

The various steps discussed above for fabricating the optical elementcan be performed by a number of suitable methods and processes. Some ofthose processes are discussed below, some of which may be related toU.S. Pat. No. 10,267,992 issued Apr. 23, 2019 to Goodno et al., titledFiber Launcher, assigned to the assignee of this application and hereinincorporated by reference.

FIG. 21 is an illustration of a 1D optical assembly 250 including anendcap 252 having machined stems 254 to which fibers 256, four of whichare shown as being representative, will be optically welded using anysuitable optically welding process, such as a plasma arc from highvoltage electrodes, CO₂ laser beams, H₂+O₂ flames, graphite filament,tungsten filament, etc., to form an emitter array 248. The assembly 250includes a stationary optical imaging system 266 that is capable ofmicro-meter level resolution that is calibrated relative to globalcoordinates represented by arrows 268. The endcap 252 is positioned on atranslational stage represented by arrows 262 to match endcapcoordinates to the global coordinates 268 by precision translation andimaging of optical fiducials associated with the endcap 252 to define animaging point 270 at the end of each stem 254 where the fiber 256 willbe welded. Prior to the fibers 256 being welded to the stems 254, thefibers 256 are mounted on a translational stage represented by arrows260, where the fibers 256 are disconnected from the translational stage260 once the fibers 256 are welded to the stems 254. An optical source258 provides optical beams 264 having optical modes propagating alongthe core of the fibers 256 that will be used to align the fibers 256 tothe stems 254 using the imaging system 266 and the imaging point 270. Aparticular fiber 256 being attached to a particular stem 254 isilluminated by the beam 264 and is moved on the stage 260 to theparticular stem 254 guided by imaging the optical beam mode locationrelative to the endcap coordinates 262. The fiber tip and the stem tipare melted locally and joined by automated jigs (not shown). After themelted joint is cooled, the mechanical hold of the fiber 256 isreleased. These steps are repeated for each fiber 256 until all of thefibers 256 are connected to the endcap 252.

Once the fibers 256 are welded to the stems 254, the next step can be todetermine position and angle errors of the fibers 256 relative to theglobal coordinates 268 by measuring the location, orientation and angleof each of the beams 264 being emitted from the fibers 256. The emitterarray 248 can be moved with precision encoders to a desired location. Inorder to obtain these measurements, a microscope 274 is used to providea near-field image 276 of the beams 264 to determine the position errorsof the beams 264 and a telescope 278 is used to provide a far-fieldimage 280 of the beams 264 to determine the pointing direction (angle)errors of the beams 264.

FIG. 22 is a front view of the near-field image 276 of the beams 264where the beams 264 are being emitted from the fibers 256 and showing anerror in the x and y direction between the center point 282 of the beam264 and an ideal x, y position 284. FIG. 23 is a front view of thefar-field image 280 showing an error in the x, y direction of the centerpoint 282 of the beam 264 relative to a target center point 286. Lenses(not shown) can be used to correct these errors.

FIG. 24 is an illustration of a 1D optical assembly 290 similar to theoptical assembly 250, where like elements are identified by the samereference number. The assembly 290 determines the position and angleerrors of the beams 264 in the near-field and the far-field when anoptical beam shaper element 292 having lenses 294, representing, forexample, one of the arrays 48 or 50, is positioned next to the endcap252. In this design, the element 292 is mounted to a translational stagerepresented by arrows 296. The beams 264 are generally output from theelement 292 as collimated beams 298. The collimated beams 298 are splitby a beam splitter 300 and one split beam is sent to image relay optics302 to generate the near-field image 276 and the other split beam isfocused by a lens 304 in the far-field to generate the far-field image280, where the near-field image 276 and the far-field image 280 are atthe image plane of the element 292 and not at the tip of the fibers 256,and where the image plane could be at the beam shaper array 50.

The position and angle errors of the beams 264 can also be measuredusing interferometry. FIG. 25 is an illustration of a 1D opticalassembly 310 illustrating this, where like elements to the assembly 290are identified by the same reference number. In this design, aninterferometer 312 is provided downstream of the beam splitter 300. Afront view of an image 316 generated at the interferometer 312 is shownin FIG. 26 , and shows an interference pattern 318 of each of thecollimated beams 298. An offset error between a center of theinterference pattern 318 and a desired position can be used to correctthe beam position and angle.

FIGS. 27A-27D are illustrations of an emitter array 320, where likeelements to the assembly 290 are identified by the same referencenumber, showing a process for correcting the beam pointing deviationangle of the fibers 256. The pointing angle error of the beam 264 ismeasured using any suitable process, such as those discussed above, asshown in FIG. 27A, to determine the direction and magnitude for bendingthe fiber 256 to make a pointing correction. The measurement is used todetermine a static force 324 to be applied to the fiber 256 somedistance away from a weld joint 322, such as tens of millimeters,between the fiber 256 and the stem 254 to bend and hold the fiber 256 inthe direction opposite to the angle error, as shown in FIG. 27B, wherethe angle error of the beam 264 does not change when the fiber 256 isbent. Localized heating 326 is applied to the stem 254 near the weldjoint 322 of the fiber 256 and the stem 254 by any suitable techniquethat is sufficient to soften the glass, but not melt it, as shown inFIG. 27C. Upon softening, the glass will deform in response to the force324 causing the beam pointing angle to change until the internalstresses created by the force 324 are relaxed. The heating is stopped,the weld joint 322 cools, and then the force 324 is removed. The glassdeformation causes the beam pointing angle to change because of thefiber angle change at the weld joint 322, as shown in FIG. 27D. Theprocess is repeated until the beam pointing direction is within acertain tolerances.

FIGS. 28A-28C are illustrations of an emitter array 330, where likeelements to the assemblies 290 and 320 are identified by the samereference number, showing a process for correcting the position error ofthe fibers 256. A beam position error 332 of the beam 264 is measuredusing any suitable process, such as those discussed above, as shown inFIG. 28A, to determine a stem shape changing direction and magnitude. Astatic force 334 by, for example, a sapphire rod having a very highmelting point and being compatible with high voltage, is applied to thestem 254 at the weld joint 322 in a direction opposite to the beamposition deviation. Because of the proximity to the high temperatureglass and the heat source, the device applying the force in physicalcontact to the stem 254 needs to be compatible with the high temperatureand high voltage local environment while not introducing contaminationto the surrounding glass. Localized heating 326, as shown in FIG. 28B,is applied to the stem 254 sufficient to soften the glass, but not reachmelting, thus enabling the stem 254 to deform by the static force. Theheating is stopped, the weld joint 322 cools, and then the force 334 isremoved. The glass deformation causes the beam position deviation tochange because of the shape of the stem 254, as shown in FIG. 28C. Thisprocess is repeated until the beam position deviation is within thedesired tolerance.

Once the proper position of the element 292 is set relative to theendcap 252 and is held there by a fixture (not shown), the element 292needs to be connected to the endcap 252 as part of, for example, theassembly 40. FIG. 29 is an illustration of a 1D optical assembly 340,where like elements to the assembly 290 are identified by the samereference number, showing the element 292 connected to the endcap 252 byopposing side bridges 342 and 344 at side surfaces of the endcap 252 andthe element 292. FIG. 30 is an illustration of a 1D optical assembly350, where like elements to the assembly 290 are identified by the samereference number, showing the element 292 connected to the endcap 252 bya top bridge 352 at top surfaces of the endcap 252 and the element 292.The bridges 342, 344 and 352 can be secured to the endcap 252 and theelement 292 by any suitable manner, such as glue, weld, diffusion bond,optical contact, etc.

Once a number of the 1D optical assemblies discussed above have beenconfigured and aligned by any of the techniques discussed above, thentwo or more of the 1D optical assemblies can be stacked on each other toform a 2D optical assembly. FIG. 31 is an illustration of a stacked 2Doptical assembly 360 including a first row 1D optical assembly 362, asecond row 1D optical assembly 364 stacked on the optical assembly 362and a third row 1D optical assembly 366 stacked on the optical assembly364, as shown, where like elements to the assembly 340 are identified bythe same reference number. A suitable precision translation table (notshown) can be employed to align the optical assemblies 362, 364 and 366as they are being assembled, and they can be secured together at theproper location by any suitable technique. A corrector lens assembly(not shown), such as the trim beam shaper arrays 62 and 66, can then beemployed to provide further beam alignment correction.

Alternately, the fibers 256 can be aligned to the endcaps 252 in each ofthe 1D optical assemblies that will be part of a 2D optical assembly,then stacked together, and then a single piece 2D optical element can bealigned to the stack. FIG. 32 illustrates an optical assembly 370 thatincludes the optical assemblies 362, 364 and 366 stacked and alignedwith each other and then aligned with a single 2D element 372, wherelike elements to the assembly 360 are identified by the same referencenumber.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present disclosure. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of thedisclosure as defined in the following claims.

What is claimed is:
 1. A coherent beam combining (CBC) fiber laseramplifier system comprising: a seed beam source generating a seed beamhaving a central wavelength and a low fill factor profile; a seed beamsplitter receiving the seed beam from the seed beam source and splittingthe seed beam into a plurality of split seed beams all having the samewavelength; a plurality of fiber laser amplifiers each receiving one ofthe split seed beams, each amplifier amplifying the split seed beam andplacing the amplified beam on a fiber; and a beam shaper array assemblyincluding an input face to which all of the fibers are opticallycoupled, said assembly further including an input beam shaper arrayhaving a plurality of input array cells positioned adjacent to and incontact with each other in a tiled manner, each input array cellincluding an input beam shaper that receives one of the amplified splitseed beams and is shaped to cause the beam to expand as it propagatesaway from the input beam shaper array, said assembly further includingan output beam shaper array having a plurality of output array cellspositioned adjacent to and in contact with each other in a tiled mannerso that each output array cell is matched to an input array cell, eachoutput array cell including an output beam shaper that receives one ofthe expanding beams and is shaped to flatten the wavefront of theexpanding beam so that the output beam shaper array provides a combinedbeam having a plurality of adjacent beams.
 2. The fiber laser amplifiersystem according to claim 1 wherein the plurality of input array cellsand the plurality of output array cells are all square cells.
 3. Thefiber laser amplifier system according to claim 2 wherein the pluralityof input array cells and the plurality of output array cells are allsquare cells of the same size.
 4. The fiber laser amplifier systemaccording to claim 2 wherein the plurality of input array cells and theplurality of output array cells are square cells of different sizes. 5.The fiber laser amplifier system according to claim 1 wherein theplurality of input array cells and the plurality of output array cellsare a combination of square cells and rectangular cells.
 6. The fiberlaser amplifier system according to claim 1 wherein the input beamshaper array and the output beam shaper array are rectilinear arrays. 7.The fiber laser amplifier system according to claim 1 wherein the inputbeam shaper array and the output beam shaper array are generally roundarrays.
 8. The fiber laser amplifier system according to claim 1 whereinthe input beam shaper array and the output beam shaper array have acenter portion with no cells.
 9. The fiber laser amplifier systemaccording to claim 1 wherein the beam shapers in the input array cellsand the output array cells are shaped in both an X and Y directionperpendicular to a propagation direction of the beams, and wherein the Xand Y direction shapes are provided on the same surface.
 10. The fiberlaser amplifier system according to claim 1 wherein the beam shapers inthe input array cells and the output array cells are shaped in both an Xand Y direction perpendicular to a propagation direction of the beams,and wherein the X and Y direction shapes are provided on differentsurfaces.
 11. The fiber laser amplifier system according to claim 1further comprising a trim beam shaper array positioned adjacent to theinput beam shaper array and receiving the plurality of expanding beams,said trim beam shaper array including a plurality of trim array cellspositioned adjacent to each other in a tiled manner, each trim arraycell including a beam shaper that receives one of the expanding beamsand is shaped to provide beam corrections.
 12. The fiber laser amplifiersystem according to claim 1 further comprising a trim beam shaper arraypositioned adjacent to the output beam shaper array and receiving thebeams therefrom, said trim beam shaper array including a plurality oftrim array cells positioned adjacent to each other in a tiled manner,each trim array cell including a beam shaper that receives one of thebeams and is shaped to provide beam corrections.
 13. A method forcoherent beam combining (CBC), said method comprising: generating a seedbeam having a central wavelength and a low fill factor profile;splitting the seed beam into a plurality of split seed beams all havingthe same wavelength; amplifying the split seed beams; causing eachamplified split seed beam to expand as it propagates to be convertedfrom the low fill factor profile to a high fill factor profile where thehigh fill factor profile tapers to a lower value at a perimeter of eachbeam; and causing a wavefront of the converted beams to flatten toprovide a combined beam having a plurality of adjacent beams withminimal overlap and a minimal gap between the beams.
 14. The methodaccording to claim 13 wherein the plurality of input array cells and theplurality of output array cells are all square cells.
 15. The methodaccording to claim 13 wherein the plurality of input array cells and theplurality of output array cells are all square cells of the same size.16. The method according to claim 13 wherein the input beam shaper arrayand the output beam shaper array are rectilinear arrays.
 17. The methodaccording to claim 13 wherein the input beam shaper array and the outputbeam shaper array are generally round arrays.
 18. The method accordingto claim 13 wherein the input beam shaper array and the output beamshaper array have a center portion with no cells.
 19. The methodaccording to claim 13 wherein the beam shapers in the input array cellsand the output array cells are shaped in both an X and Y directionperpendicular to a propagation direction of the beams, and wherein the Xand Y direction shapes are provided on the same surface.
 20. The methodaccording to claim 13 wherein the beam shapers in the input array cellsand the output array cells are shaped in both an X and Y directionperpendicular to a propagation direction of the beams, and wherein the Xand Y direction shapes are provided on different surfaces.